Middle Grey
The reference tone, reflecting about 18 per cent of the light that strikes it, that every reflected-light meter is calibrated to reproduce.
Formula / rule
L* ≈ 50
Measured in
% reflectance
In depth
Middle grey sits halfway between black and white in perceptual terms, not in linear light — which is why 18 per cent reflectance rather than 50 per cent is the standard. Point a meter at anything and it will give the settings that render that thing as middle grey; every exposure compensation decision is really a statement about how far your subject sits from this tone.
Examples in practice
Weathered tarmac, dry grass in soft light and the palm of an average hand are all close enough to middle grey to meter from directly.